Discover the fascinating world of genetic genealogy! Written for the non-scientist, YGG is the best source for unbiased news on the major genealogy DNA testing companies.
Written by CeCe Moore, an independent professional genetic genealogist and television consultant.

Sunday, July 24, 2011

Known Relative Studies with 23andMe: More Second Cousin Comparisons

Since my last post on second cousin comparisons at 23andMe was so popular, I have decided to expand on it. Another second cousin of mine recently tested at 23andMe. As a result, I have many more relevant comparisons to share with my readers. This cousin is also from my mother's family, but not on her Finnish side. Our common ancestral couple is Daniel Hewitt and Amelia (Travis) Proctor, my great grandparents. Daniel was born in 1866 in Equality, Illinois and Amelia "Millie" was born in 1870 in Sidney, Iowa.

Again, this is a great example of the random nature of autosomal DNA inheritance. My two sisters share 2.56% and 2.98% of their DNA with our Proctor second cousin, while I only share 1.07% of my DNA with this cousin. (The expected percentage of shared DNA with a second cousin would be an average of 3.125%.) Our unique patterns of inheritance can be clearly seen in the chart below. The long gray bars are representative of the 23 chromosomes and the shorter colored blocks signify the shared DNA between each person tested and our Proctor cousin. (For example, toward the end of Chromosome 1, Sister #1 and Sister #3 share the same small stretch of DNA with our Proctor cousin, but Sister #2 does not share any DNA with this cousin until Chromosome 5.)

Click on chart to enlarge

You can really see here that we inherited very diverse blocks of our Proctor great grandparents' DNA. One of my sisters inherited almost two and a half times more of this "Proctor" DNA than I did, while my other sister inherited nearly three times more than I did. In fact, 23andMe predicts that we are only third cousins, but correctly predicts both of my sisters' relationships to this second cousin.

From this comparison, it would appear that my sisters ended up with significantly more DNA from our Proctor great grandparents than I did. However, since this chart really just illuminates the shared Proctor DNA with this particular cousin, it could be that my sisters happened to inherit much of the same DNA from Dan and Millie that our cousin did and I inherited almost entirely different blocks. To be sure, I would need to test more cousins descended from these ancestors to see if my comparatively low percentage of sharing continues.

This chart shows how my mother compares to her Proctor first cousin once removed (4.56%) and the DNA inheritance patterns of two of us daughters. You can see that the DNA was passed on quite differently to each with only three small areas of overlap between all three.

The chart below shows that my mother's sister inherited significantly more DNA in common with their Proctor first cousin once removed than my mother did: 4.56% vs. 8.55%.

Based on this, it isn't surprising that my aunt's two daughters in the chart below share much more DNA with this cousin than my sisters and I do (#1 = 4.49% and #2 = 4.69%) and with much larger areas of overlap between the three.

It is always interesting to me to see the inheritance pattern as the DNA is passed through the generations. The following charts demonstrate this with each comparing three successive generations to our Proctor cousin.

Lastly, the third generations in the charts above are second cousins once removed from this Proctor cousin. In the charts below, you can see, once again, that the amounts of common DNA inherited at this level of cousinship vary widely with very little overlap. (The only difference between the following two charts is the dark blue comparisons.)

The percentages of DNA in common for the second cousins once removed in this study were 2.91%, 1.22%, .76% and .33% (expected average = 1.563%). For second cousins, the percentages were 4.69%, 4.49%, 2.98%, 2.56% and 1.07% (expected average = 3.125%).

5 comments:

Very interesting. I think testing second cousins and half cousins ends up being more valuable in phasing data than with first cousins. It is possible for us to determine a narrower inheritance path. I have another second cousin in the cue to be tested (number 3). She should provide some interesting results.

I've got a paper trail 2nd cousin who shares 1.23% over 6 segments. 23and me says she is most likely a 3rd cosuin, range 2nd-3rd. My suspicious mind says she could be somewhere in between. Yet to put my mind to that issue of what cousinship that is if my grandmother's much older sister was her actual mother which is what I have thought for a long time now. If that's the case that would certainly help to explain why none of her father's ancestors have shown up and my abundance of Rhineland folk on RF and AF.

Since second cousins once removed would be expected to average 1.563% of their DNA in common that could indeed be what you are seeing in your family. However, the cousin above is definitely my second cousin and I only share 1.07% of my DNA with her, while my first cousins who share the same relationship with her have 4.49% and 4.69% in common with her. This tells us that the inheritance at this level is already so random that the actual percentages can vary widely and one cannot read too much into a low sharing with one member of the family without testing other family members.